In the attachment there is a diagram of how they do it in the US to the drug, as is the case with approvals for other rallies. They disconnect a plus.
Although who I do not talk to, he advises to disconnect the minus ...
I also noticed that quite often
the alternator is connected to the battery directly in cars of different manufacturers.
Unfortunately, I do not know and do not understand for what purpose.
Unfortunately, I can not find any schematic for audi or bmw how they solve it.
Because maybe the additional line is just for the alternator.
As for the cable cross-section, unfortunately I do not know what the cross-section of this cable is,
but it is used in many Audi models, it has its length so I think the manufacturer
took into account its size to length.
As for disconnecting the mass instead of a plus, I would have to lead a second, quite long cable if it was to be disconnected with a manual switch. (shorter if the magnetic relay is located in the trunk)
What is safety in motorsport is usually detached outside the car,
and in commercial vehicles, the pyrotechnic clamp disconnects immediately at the battery for safety.
I, on the other hand, want to disconnect in the cabin due to the fact that I modified quite a lot
the entire installation and I want to be in control in case something is fried.
Therefore, the switch should be in the cabin.
In fact, disconnecting ground seems to be the better solution.
There is no live wire left, I will add a wire going under the carpet directly on
car body. But the problem is that the positive wire would be almost 5 meters and the negative wire would be almost 3 meters instead of one.
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To understand the matter, the most I would like to know is whether the starter current with two wires, one thick, the other 2/3 thinner, will not overheat the thinner one, or the percentage
it will be distributed according to the resistance of the cables and the thinner wire is safe ??
. Below are 3 variants of how I would do it: 
